1. Field of the Invention
The present invention relates to a hybrid vehicle having both an internal combustion engine and an electric motor for driving and, more particularly, to a technique for detecting both a failure in an ignition system (ignition system failure) of an internal combustion engine mounted on the hybrid vehicle and a failure in a rotation sensor system of the internal combustion engine.
2. Description of Related Art
Hitherto, a vehicle is driven by transmitting a rotation output generated by an internal combustion engine to wheels (driving wheels) and exhaust gases and noises occur upon driving. An electric vehicle driven by an electric motor has therefore been proposed. The electric vehicle, however, has a problem that a travel distance must be short because only an electric power preliminarily charged in a battery is used.
In recent years, a hybrid vehicle using both an internal combustion engine and an electric motor for driving has been used in practice used. In the hybrid vehicle, a generator is driven by an output of the internal combustion engine, power generated by the generator is charged into a battery, an electric motor is rotated by the electric power charged in the battery or the electric power from the generator, and the rotation output of the electric motor is transmitted to driving wheels.
Various types of the hybrid vehicles of this kind have been proposed such as a series type in which the internal combustion engine drives only the generator and the wheels are driven only by the electric motor and a parallel-series type which can apply a driving force to the wheels by both of the internal combustion engine and the electric motor. In the series hybrid vehicle (hereinbelow, referred to as an "SHV"), the internal combustion engine is mechanically separated from the driving system. When power generation is considered to be unnecessary in the cases such as a vehicle halt (the vehicle speed=zero) and during a driving in a state where the battery is sufficiently charged, the operation of the internal combustion engine is stopped. When a charge amount of the battery becomes lower than a predetermined amount in association with driving, the internal combustion engine is started by a separately provided starting motor or the like and is made operative and the battery is charged.
A parallel-series hybrid vehicle (hereinbelow, referred to as a "PSHV") has a power transmitting mechanism such as a magnet clutch and a planetary gear unit for transmitting both an output of the internal combustion engine and an output of the electric motor to the drive shaft for driving the wheels. Since the output of the internal combustion engine can be used not only for power generation but also for directly driving the wheels together with the output of the electric motor, energy transmission efficiency is high and the PSHV is considered to be especially promising.
In the PSHV, usually, the operation of both of the internal combustion engine and the electric motor is stopped when the vehicle comes to a halt. When the driver operates the accelerator pedal, only the electric motor is driven first to start the vehicle. In such a state, the internal combustion engine is in a "motoring state" where the fuel is not injected and the engine is rotated by the output of the electric motor. In the state where the fuel is not injected to the internal combustion engine, ignition to the engine is also stopped. After that, when the vehicle speed exceeds a predetermined value (for example, 20 km/h), the fuel injection and the ignition to the internal combustion engine are started and the internal combustion engine starts in a manner, i.e., the so-called "pushed start." After that, both the output of the internal combustion engine and that of the electric motor are controlled according to the driving load of the vehicle and the charging state of the battery. When the vehicle speed becomes below the predetermined value, the fuel injection and the ignition to the internal combustion engine are stopped, the internal combustion engine enters the "motoring state" again, and the vehicle is driven by using only the output of the electric motor.
In the internal combustion engine, an ignition system for igniting the fuel and a rotation sensor system are mounted. When a failure occurs in the ignition system, unburned gases are released from an exhaust pipe to the atmosphere in association with the fuel injection. When a failure occurs in the rotation sensor system, injection and ignition timing is deviated, so that unburned gases are released and the drivability of the vehicle deteriorates. Consequently, hitherto, in a control unit for controlling the internal combustion engine and the electric motor of the hybrid vehicle of this kind, in a manner similar to a common vehicle having only the internal combustion engine, whether or not the ignition system normally operates according to an ignition instruction signal generated from the control unit itself is monitored in the engine operating state where the fuel injection and the ignition to the internal combustion are performed. Whether a pulse signal is normally generated from the rotation sensor system or not is also checked. When a failure is detected by such failure diagnosis, the fuel injecting operation and the igniting operation are stopped to thereby stop the operation of the internal combustion engine.
The conventional control unit is, however, insufficient to achieve the inherent object of the hybrid vehicle, namely, suppression of environmental pollution. More specifically, in the conventional control unit, since failure diagnosis is performed to both the ignition system and the rotation sensor system when the internal combustion engine is in the operating state, deterioration in the exhaust gases (release of unburned gases) cannot be avoided for a period of time from the detection of a failure until the stopped of the fuel injection. Moreover, in the failure diagnosis of this kind, generally, occurrence of a failure is determined for the first time after a certain number of failures are detected in order to avoid erroneous detection. Consequently, due to the delay in the failure detection, unburned gases are released more.
Especially, in case of the PSHV, the output shaft of the internal combustion engine and that of the electric motor are mechanically connected via the power transmitting mechanism. The internal combustion engine is started in a "pushed start" manner by injecting fuel and igniting thereto at proper timing which matches the motoring speed. Upon starting the internal combustion engine in the PSHV, a larger amount of fuel is injected. Therefore, when the ignition system or the rotation sensor system is faulty, a larger amount of unburned gases is released, and the drivability is also adversely affected. Moreover, the emission of the unburned gases may cause an irreversible damage to a catalytic converter mounted on the vehicle for purifying the exhaust gases.